Abstract Proper control of serum phosphate concentrations is required to maintain skeletal integrity. We previously identified missense mutations in Fibroblast growth factor-23 (FGF23) as the cause of autosomal dominant hypophosphatemic rickets (ADHR), characterized by hypophosphatemia secondary to isolated renal phosphate wasting and metabolic bone disease. We subsequently demonstrated that inactivating FGF23 mutations result in the mirror-image disorder to ADHR, hyperphosphatemic familial tumoral calcinosis (hfTC), manifested by markedly elevated serum phosphate and often severe ectopic and vascular calcifications. Further, FGF23 is elevated in X-linked hypophosphatemic rickets (XLH) and increased circulating FGF23 is also associated with a 5-6 fold higher mortality risk in patients with chronic kidney disease-mineral bone disorder (CKD-MBD). There are currently no cures for the metabolic bone disease, only maintenance treatments, for the aforementioned syndromes. Although much progress has been made towards understanding both basic and clinical aspects of phosphate metabolism, the fundamental mechanisms regulating FGF23 bioactivity under normal conditions and in disease are unknown. Certainly, the discovery of the FGF23 co-receptor ?-Klotho (?KL) was a major step forward in our understanding of FGF23 actions in target tissues, however the downstream pathways directing phosphate metabolism in kidney, and thus overall bone health remain unclear. These exploratory studies are based upon initial, exciting findings derived through an unbiased approach, and will test the hypothesis that specific intermediaries are upregulated by FGF23 and control phosphate handling in situations of clamped serum FGF23 and in the absence of ?KL-mediated signaling. The specific aims undertaken herein will examine pathways new to the phosphate field, thus having the potential to produce significant impact as a means to potentially control phosphate handling in diseases of FGF23 excess or absence. A dovetailed combination of ex vivo and in vivo approaches will be undertaken to examine a novel functional FGF23 axis, and this proposal will also explore direct inhibition of FGF23 bioactivity through previously unrecognized pathways in mouse models of genetic hypophosphatemia. It is anticipated that successful completion of this work would provide important targets for rare and common disorders of phosphate handling.